This invention relates to ceramic and metal containing parts that are formed from powder, and more particularly to such parts made from fine powder. Structural ceramic parts are generally made of fine powders, typically on the order of 1 micron in size. Such powders are difficult to spread dry, as they tend to agglomerate. Use of such fine powders offers several advantages. First the use of fine powders enhances sinterability. Sintering is a solid state diffusion process and will be enhanced by increasing the surface area to volume ratio of the powder in any green part that is subsequently sintered. As is known, surface roughness can be no smaller than the powder size. Thus, using fine powders also enables overall part quality to be improved. Smaller powders also mean that the minimum feature size that can be specified is also improved. Lastly, smaller powders allow thinner layers to be used in any layered fabrication technique, which helps eliminate slicing defects such as stair-stepping.
A processing technique that uses powders has become known as xe2x80x9cthree-dimensional printingxe2x80x9d (xe2x80x9c3D Printingxe2x80x9d) and is described in general in numerous patents, including: U.S. Pat. No. 5,204,055, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,340,656, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,387,380, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Cima, Sachs, Fan, Bredt, Michaels, Khanuja, Lauder, Lee, Brancazio, Curodeau, and Tuerck; U.S. Pat. No. 5,490,882, entitled PROCESS FOR REMOVING LOOSE POWDER PARTICLES FROM INTERIOR PASSAGES OF A BODY, by Sachs, Cima, Bredt, and Khanuja; and U.S. Pat. No. 5,660,621, entitled BINDER COMPOSITION FOR USE IN THREE-DIMENSIONAL PRINTING, by James Bredt. All of the foregoing 3D Printing patents are incorporated herein fully by reference.
3D Printing is also disclosed and discussed in, co-assigned applications, including: U.S. Ser. No. 08/600,215, filed Feb. 12, 1996, entitled CERAMIC MOLD FINISHING TECHNIQUES FOR REMOVING POWDER, by Sachs, Cima, Bredt, Khanuja, and Yu; U.S. Ser. No. 08/596,707, filed Feb. 5, 1996, entitled HIGH SPEED, HIGH QUALITY THREE DIMENSIONAL PRINTING, by Sachs, Curodeau, Fan, Bredt, Cima, and Brancazio; U.S. Ser. No. 08/856,515, filed May 15, 1997, entitled CONTINUOUS INK-JET DROPLET GENERATOR, by Sachs and Serdy; U.S. Ser. No. 08/551,012, filed Oct. 31, 1995, entitled ENHANCEMENT OF THERMAL PROPERTIES OF TOOLING MADE BY SOLID FREE FORM FABRICATION TECHNIQUES, by Allen, Michaels, and Sachs; U.S. Ser. No. 08/831,636, filed Apr. 9, 1997, entitled THREE DIMENSIONAL PRODUCT MANUFACTURE USING MASKS, by Sachs and Cima; and U.S. Ser. No. 60/060,090, filed Sep. 26, 1997, entitled REACTIVE BINDERS FOR METAL PARTS PRODUCED BY THREE DIMENSIONAL PRINTING, by Sachs, Yoo, Allen, and Cima (provisional application). All of the foregoing 3D Printing patent applications (and provisional application) are incorporated herein fully by reference.
Basically, the 3D Printing process is a Solid Freeform Fabrication (SFF) process, which allows parts to be created directly from computer models. Other SFF processes that are commonly used include stereolithography (SLA), selective laser sintering (SLS), laminated object manufacturing (LOM), and fused deposition modeling (FDM). These processes all differ from traditional machining, since material is added to the desired part, as opposed to material removal in milling, turning, and boring.
A typical implementation of the 3D Printing process begins with the definition of a three-dimensional geometry using computer-aided design (CAD) software. This CAD data is then processed with software that slices the model into many thin layers, which are essentially two-dimensional. A physical part is then created by the successive printing of these layers to recreate the desired geometry. An individual layer is printed by first spreading a thin layer of powder and then printing binder to adhere the powder together in selected regions to create the desired layer pattern. The growing part is lowered by a piston and a new layer of powder is spread on top. This process is repeated until all the layers have been printed. The binder joins powder together within a layer and between layers. After printing is complete, the unbound powder is removed, leaving a part with the desired geometry. Typically the part is a green part that will experience further processing, such as sintering. However, in some circumstances, the part may be a final part.
This traditional 3D Printing layer generation technique relies on the powder being flowable in order for smooth layers of uniform density to be created.
There are many different powder and binder systems, based on metal, or ceramic or polymer powder. The part can be sintered or infiltrated to full density. Because 3D Printing is an additive manufacturing process, many geometries are possible that are not feasible with traditional machining, such as undercuts and internal cavities. Furthermore, many materials can be used in the 3D Printing process, as long as they can be obtained in powdered form. Currently, work has been done using metal, polymer, ceramic, and glass-ceramic powders. Using these materials, a wide variety of parts have been produced. This includes the direct printing of metal parts, injection molding tooling, casting shells, and structural ceramics. Parts, such as tooling, can incorporate conformal cooling channels to surfaces to decrease cycle time and residual stresses in parts made with such tooling. Other types of parts can also include such channels. Using the 3D Printing process, it is also possible to make individual parts with regions composed of varying materials (functionally gradient materials). This can be achieved by printing different materials into selected regions of an individual layer. This extra degree of freedom allows designers to vary the material properties within a single part.
Despite the many advantages of using fine powders mentioned above, they are difficult to use in the known 3D Printing process for a variety of reasons. Fine powder particles tend to stick to each other, forming agglomerates, due to various reasons, including Van der Waal""s attractive forces, and moisture. The particles also tend to stick to any other bodies they come into contact with, including powder piston walls and the powder spreader bar. Low flowability also occurs because very fine particles are typically irregularly shaped, increasing friction. Poor flowability combined with powder adherence to the spreader bar makes it difficult to spread smooth layers. The low flowability of the powder also inevitably leads to uneven densification within layers and consequently, the resulting green body. Further, it is difficult to print into fine powders, without problems of ballistic ejection and erosion due to the binder jet impinging on the power bed surface.
Another problem with forming parts from powder relates to packing density and sintering. Much work has been done with spherical granules, typically ranging from 30-100 xcexcm in size. The granules are actually agglomerates of submicron powder bound together-by an organic phase with a typical packing density of about 50% of theoretical. As a result, the packing density of the resulting green part is too low to be sintered directly (typically 30-35% of theoretical). An iso-static pressing step is required to increase the green body packing density. After iso-static pressing, alumina green parts fabricated with spray dried powders exhibit packing densities ranging from 59-63% of theoretical, depending on whether the cold iso-static pressing (CIP) or the warm iso-static pressing (WIP) process is used. This is adequate to achieve full density during sintering.
However, use of these processes introduces several problems. Before sintering, green parts are quite fragile and easy to damage. Pressing requires a relatively large amount of part handling. Further, the pressing step can introduce density gradients within the green part. This can cause problems during sintering, such as warping and anisotropic shrinkage. A final disadvantage of the iso-static pressing step is that it raises production costs.
Attempts have been made to eliminate the iso-static pressing post processing step, by increasing the printed part packing density. It has been suggested to print slurry of the desired material into spray dried granules, in order to increase the powder bed packing density. However, that process also has drawbacks.
Accordingly, for the foregoing reasons, there is a need for processes that allow the formation of powder beds with uniform properties, that are made up of fine powders. Techniques are also required to handle fine powder that overcomes the problems of agglomeration and poor flowability experienced with dry powders. There is also a need for a process to establish a fine powder bed to enable using 3D Printing with fine powders, to minimize or eliminate problems of ballistic disturbance. Similarly, there is a need to develop a high density powder bed. An additional need is to create parts from powder beds that can be directly sintered, or which can be sintered without deleterious pre-sintering steps.
Thus, the several goals and objects of the invention include to form beds of fine powder with uniform properties. Another object of the invention is to enable handling fine powders without undue agglomeration, and with acceptable flowability. A further object of the invention is to enable 3D printing using fine powder beds, resulting in smooth surfaced objects having very fine features and controllable dimensions. Yet another object of the-invention is to facilitate fabrication of parts from fine powder in high density beds, using 3D Printing, which parts can be sintered directly from green parts, without any difficult preparatory pre-sintering steps.
In general, according to the present invention, a powder bed is built up by repeated deposition of a slurry that contains the powder. Layers of powder are made by depositing a liquid dispersion of the desired powdered material, which then slip-casts into the forming powder bed to make a new layer. Fine powder beds can thus be made without the flowability problems associated with dry powder handling discussed above. The slurry may be deposited in any suitable manner, including depositing in separate, distinct lines, such as by raster or vector scanning, or by a plurality of simultaneous jets that coalesce before the liquid slip-casts into the bed, or by individual drops, the deposit of which are individually controlled, thereby generating a regular surface for each layer. Liquid is removed from the bed first by slip-casting and then by drying, such as with heat, after deposition of each layer. The powder bed can be used for various processes. If the 3D Printing process is being used, then after each layer of powder is jetted and dried, the next step is to print a pattern of binder for the layer under construction. There may be a drying step after the binder deposition. These steps of slurry jetting, liquid reduction, (slip-casting; drying) binder deposition, and (optionally) binder drying, are repeated until the desired number of binder patterned layers have been built up. The intermediate product is at this point a block of powder. The unbound powder is dispersed, typically by immersion in a solvent (water is typical). The resulting part is typically a green part suitable for densification, either by sintering or infiltration, as is known, with care taken to accommodate the binder/powder system chosen.
A preferred embodiment of the invention is a method for creating a powder containing body. The method includes the steps of: providing a support; over a selected area of the support, depositing in a continuous stream a liquid slurry that contains a first powdered material to form a first layer of powdered material; maintaining the deposited layer region of powdered material under conditions such that the liquid content of the first layer is reduced; and over a portion of the selected area of the support, depositing in a continuous stream the liquid slurry to form an additional layer. Typically, the method includes many repetitions of the steps of maintaining and depositing the slurry until a desired thickness of powdered material has been deposited. Reduction of the liquid content is typically effected by allowing the liquid of the slurry to slip into porosities of a previously deposited layer. Subsequent drying, for instance, by heating, is also typically practiced.
The step of depositing the slurry may be accomplished by rastering a jethead over a selected area, and jetting slurry while rastering. Rather than rastering, vectoring motion can also be used. Alternatively, a jethead can be passed over the portion of the selected area while simultaneously jetting a plurality of parallel streams of the slurry from the jethead onto the portion of the selected area.
If a plurality of streams are deposited, it is particularly beneficial to jet them onto the portion of the selected area such that each of the streams is deposited closely enough to an adjacent stream in space and time such that, upon contact with a previously deposited layer, liquid from adjacent streams merges before the liquid has completely slipped down into porosities of the previously deposited layer. Typically, when using a plurality of streams, they are spaced apart a distance of between 1.5 and 6.0 diameters of the streams, and preferably between 2 and 4 diameters.
Such timing and placement can also be accomplished by serial application of streams, either by rastering, or by a rotating arrangement, where relative rotation is set up between a support and a slurry deposition unit. Either the support or the deposition unit can be moved.
The slurry may contain particles of between 0.2 and 10.0 microns, preferably of between 0.5 and 2 microns.
The slurry may be deposited through a nozzle having a greatest orifice dimension of between 50 and 1000 microns and preferably between 100 and 400 microns.
The orifice(s) may be circular in cross-section, or elongated. If elongated, typically the aspect ratio is greater than 3:1.
The slurry typically has a solids volume fraction of between 5 and 55, and preferably between 10 and 40.
The method of the invention can be practiced using powders of metal, ceramic and polymers. More than one type of slurry may be used.
An additional aspect of the invention is to measure the height of the deposited additional layer at selected locations of the selected area and to adjust the rate of delivery of slurry at selected locations of subsequently deposited layers, based on the measured height at the selected locations of the additional layer. The rate of delivery can be adjusted by adjusting the speed by which the jet head passes over the surface. If a plurality of streams are deposited simultaneously, the rate of delivery of slurry among the plurality of parallel streams can be varied to correct for any irregularity in surface height.
Lines of slurry of a subsequent layer can be jetted in register or offset from lines of slurry of a preceding layer. Further, a first set of spaced apart streams of slurry can be deposited, with a second set of streams being deposited subsequently, each stream of the second set being deposited between a pair of adjacent spaced apart streams of the first set.
Another preferred aspect of the invention is to deposit additional layers of slurry of a thickness such that that a film forms, including the deposited slurry and previously deposited powder, the film having a saturation thickness, hsat, that is less than a critical cracking thickness CCT. In addition, it is an aspect of the invention to avoid layer cracking by adjusting at least one of the following factors as indicated: increasing the volume fraction VF of the slurry; decreasing the surface tension, xcex3LV, of the slurry; increasing the fracture resistance, Kc of the film; increasing the contact angle, xcex8, of the slurry on the solid phase; and increasing the pore radius of the film.
According to another embodiment of the method of the invention, the invention is a method for creating a powder containing body, including using a binder. This embodiment of the invention includes the steps of providing a support; over a selected area of the support, depositing a liquid slurry that contains a first powdered material to form a first layer of powdered material; maintaining the deposited layer of powdered material under conditions such that the liquid content of the first layer is reduced; depositing a binder material at selected regions of the layer, which binder will cause the layer to become bound at the selected regions, and successive layers to become bound to each other at the selected regions; and over a portion of the selected area of the support, depositing liquid slurry to form an additional layer region.
The binder can be printed or deposited through a mask. The slurry can be deposited as a continuous stream, or as individually controlled droplets.
Typically, this embodiment of the method includes further the steps of repeating the maintaining, depositing a binder material and depositing slurry steps a plurality of times until a desired thickness of powdered material has been deposited.
All of the other variations and features mentioned above, in connection with making a three dimensional object, without regard to whether a binder material is provided, are also applicable to the embodiment of the invention in which a binder is used. There are also additional features.
A preferred aspect of the invention is to include a redispersant agent in the slurry composition. Preferably, the redispersant is: soluble in the liquid used to form the slurry; soluble in a liquid medium that can redisperse the formed layers of powder; and soluble in any liquid vehicle of the binder material. Further, it is a preferred embodiment of the invention for the redispersant to be substantially the last of any material of the slurry to slip cast during drying, such that residues thereof form at necks between powder particles.
According to yet another preferred embodiment, the binder should penetrate into pores of the powder bed; bind the powder in the presence of any re-dispersing agent; and be insoluble in any liquid used for re-dispersion of any unbound powder. The binder may be a heat curable polymer, and the method of the invention may include a step of heat treating the formed powder bed to cure the binder.
Still another aspect of this embodiment of the invention is that, after the desired number of jetted layers of powder with printed regions of binder has been produced, unbound powder is dispersed from bound powder, such as by contacting the unbound powder with a solvent.
The method may be practiced with a non translating support, over which a slurry deposition unit and a binder deposition unit pass, repeatedly, layer after layer. Two or more of each can be incorporated into the same, stationary set up. Alternatively, the support can move from one stage to another, each stage having its own slurry deposition unit(s) and binder deposition unit(s). Such a production line set up can follow any path, including a linear, or cyclic path.
Yet another preferred embodiment of the invention is a process for making a component comprising the steps of: depositing a layer of a powder material in a region by jetting a slurry that contains the powder material; applying a further material to one or more selected regions of the layer of powder material which will cause the layer of powder material to become bonded at the one or more selected regions; repeating the layer depositing step and further material application steps a selected number of times to produce a selected number of successive layers, the further material causing said successive layers to become bonded to each other; and removing unbonded powder material which is not at said one or more selected regions to provide the component.
Another preferred embodiment of the invention is an apparatus for making three-dimensional objects comprising: a support having a surface; a slurry delivery unit; a slurry delivery drive unit, configured to drive the slurry delivery unit to deposit a layer of slurry over a selected region of the surface; a liquid reduction unit arranged to reduce the liquid content of a layer of deposited slurry; a binder delivery unit; and a binder delivery drive unit configured to deposit binder material at selected regions of a layer of deposited slurry.
According to a preferred embodiment, the slurry delivery unit has a nozzle, which may have an orifice having a greatest opening dimension of between 50 and 1000 microns, and preferably between 100 and 400 microns.
The slurry delivery unit may also comprise a head having a plurality of spaced apart orifices, of the same size range, which may be spaced apart from each other between 1.5 and 6 diameters of a slurry stream jetted from an orifice, and preferably between 2 and 4 diameters.
According to still another preferred embodiment, the slurry deliver drive unit is configured to raster the slurry delivery unit over the surface of the support. Alternatively, it may be configured to sweep or vector the slurry delivery unit across the surface of the support.
Yet another preferred embodiment of the invention includes a layer surface height measurement unit and a surface height control module that takes as an input a signal from the surface height measurement unit and uses that signal to vary the delivery of slurry so as to control the height of the surface that is formed. The surface height measurement unit may comprise a laser rangefinder.